The present application claims priority under 35 U.S.C. xc2xa7119 to Korean Application No. 2000-66831, which is hereby incorporated by reference in its entirety for all purposes.
1. Field of the Invention
The present invention relates to a method of forming patterns, and more particularly, to a method of forming patterns using two alternating phase shift masks.
2. Description of the Related Art
It is well known that patterns used in manufacturing a semiconductor device are usually formed by photolithography. In photolithography, a photoresist whose solubility varies is formed on a layer where a pattern will be formed, such as an insulating layer or a conductive layer on a semiconductor substrate. The solubility of the photoresist is varied by irradiating light such as x-rays and ultraviolet rays thereon. For example, a prescribed region of the photoresist is exposed to light using a mask, and subsequently a part of the photoresist which has a high solubility with respect to a developing solution is removed, thereby forming a photoresist pattern. After the photoresist pattern is formed, the exposed portion of the underlayer where the pattern will be formed is removed by an etching process, thereby forming various patterns such as an interconnection and an electrode.
As semiconductor devices become more highly integrated, a lithography technology which can form much finer patterns is needed. In response to this requirement, an exposure method using an electron beam, an ion beam or x-rays; a modified illumination method using the diffraction of a light source; an exposure method using new photoresist materials; and an exposure method using a phase shift mask have been suggested.
Among these methods, the exposure method using a phase shift mask, can increase resolution or depth of focus by installing a shifter in the phase shift mask and using whole or partial interference of rays. In other words, when light passes through a mask substrate of the phase shift mask or a shifter layer, its wavelength shortens to the quotient of the wavelength in a vacuum divided by a refractive index. Thus, the phase of light can be varied depending on whether there is a shifter or not.
Here, if xcex8 indicates the path difference of light, xcex8=2xcfx80t(nxe2x88x921)/xcex (wherein n is the refractive index of the shifter, t is the thickness of the shifter, and xcex is the wavelength used). If xcex8 is equal to xcfx80, light which has passed through the shifter has an inverse phase. Therefore, if the shifter is placed at the edge of a mask pattern, the contrast of light may be increased because light passing through a transparent region has an inverse phase with respect to that of light passing through the shifter, and consequently the intensity of light becomes zero at the boundary of the pattern. Unlike other exposure methods, the exposure method using the phase shift mask can greatly improve the resolution of the mask by changing only a mask manufacturing method, without changing equipment or photoresist materials.
Various types of phase shift masks are manufactured. For example, an alternating phase shift mask includes two different types: a glass substrate etching type which can shift a phase 180xc2x0 by etching a glass substrate, and a SOG type which can shift phase 180xc2x0 by covering a common mask substrate with SOG and subsequently patterning. Hereinafter, a method of forming a pattern using a conventional alternating phase shift mask of the glass substrate etching type, will be described.
FIG. 1(a) is a sectional view of an example of a conventional alternating phase shift mask, and FIG. 1(b) is a graph illustrating the distribution of light intensity on a wafer during exposure using the alternating phase shift mask of FIG. 1(a). In FIG. 1(a), a plurality of opaque patterns 13 which shield light are formed on the surface of a glass substrate 11 of an alternating phase shift mask 10. Each opaque pattern 13 is made of a chrome layer. Between two opaque patterns 13, there is a groove 15 formed by etching the glass substrate 11. Each groove 15 acts as a phase shifter. The phase of light passing through the grooves 15 is inverted by 180xc2x0 with respect to the phase of light passing through the other portions, thereby enhancing the resolution. Therefore, the alternating phase shift mask 10 of FIG. 1(a) includes non-transparent regions (NT), phase shift regions (S), and non-phase shifted regions (NS).
During exposure using the alternating phase shift mask illustrated in FIG. 1(a), exposure light passing through the phase shift region loses its energy because the light is scattered between both side walls of each groove 15. Therefore, as illustrated in FIG. 1(b), the intensity of exposure light passing through the phase shift regions (S) becomes smaller than the intensity of light passing through the non-phase shifted regions (NS). Consequently, there is a difference in the critical dimension (CD) of photoresist patterns formed on a wafer, and this is referred as a xcex94CD phenomenon.
FIG. 2(a) is a sectional view of another example of a conventional alternating phase shift mask, and FIG. 2(b) is a graph illustrating the distribution of light intensity on a wafer during exposure using the alternating phase shift mask of FIG. 2(a). The same reference numerals in FIGS. 1(a) and 1(b) represent the same elements. Except for the formation of undercut grooves 25 as a shifter, the alternating phase shift mask 20 of FIG. 2(a) is the same as that of FIG. 1(a). The grooves 25 are formed by etching portions of the glass substrate 11 under the opaque patterns 13. The undercut grooves 25 can significantly reduce the difference of light intensity between a phase shift region (S) and a non-phase shifted region (NS) during exposure, thereby essentially preventing the xcex94CD phenomenon. However, there are problems when using the alternating phase shift mask of FIG. 2(a), as described with reference to FIGS. 3 and 4.
FIGS. 3 and 4 are graphs illustrating the distribution of light intensity with respect to position on a wafer and the critical dimension on a wafer with respect to different exposure focuses, respectively, using the alternating phase shift mask illustrated in FIG. 2(a). During exposure using the alternating phase shift mask 20 in which the undercut grooves 25 of FIG. 2(a) are used as shifters, when the focus is 0 xcexcm as illustrated in FIGS. 3 and 4, there is no difference in the light intensity and in the critical dimension respectively between a phase shift region (S) and a non-phase shifted region (NS). On the other hand, when the focus is not 0 xcexcm, the critical dimensions of a phase shift region (S) and a non-phase shifted region (NS) are very different from each other due to the difference of light intensity between them.
Moreover, when the focus changes from xe2x88x920.4 xcexcm to 0.4 xcexcm, the line indicating the critical dimension of the non-phase shifted region (NS) has a positive gradient, but the line indicating the critical dimension of the phase shift region (S) has a negative gradient. These two lines cross each other at a particular focus. Therefore, as the focus changes from xe2x88x920.4 xcexcm to 0.4 xcexcm, the critical dimension of the phase shift region (S) inverses that of the non-phase shifted region (NS), on the basis of the focus of 0 xcexcm. This is referred as an xe2x80x9cinversion phenomenon of critical dimensionsxe2x80x9d.
However, since the alternating phase shift mask at which the inversion phenomenon of critical dimensions occurs has small mask or phase margin, it cannot be used at all or its usefulness is quite low. Moreover, the alternating phase shift mask of FIG. 2(a) has another problem in which the opaque patterns 13 are damaged during the formation of the undercut grooves 25.
The present invention is therefore directed to a pattern formation method using two alternating phase shift masks, which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
To solve the above and other problems, it is an object of the present invention to provide a method of forming patterns by preventing an inversion phenomenon of critical dimensions, increasing the resolution of an alternating phase shift mask, and enlarging focus margins.
To achieve the above and other objects of the invention, the present invention provides a method of forming patterns in which, after a first exposure has been performed using a first alternating phase shift mask, a second exposure is subsequently performed using a second alternating phase shift mask.
The first alternating phase shift mask includes a first non-phase shift region where exposure light is transmitted and the light phase is not changed, a first phase shift region where light is transmitted and the light phase is changed, and a non-transparent region through which light cannot penetrate, located between the first non-phase shift region and the first phase shift region, on a first mask substrate, that is a glass substrate.
The second alternating phase shift mask includes a second non-phase shift region where light is transmitted and the light phase is not changed, a second phase shift region where light is transmitted and the light phase is changed, and a non-transparent region through which light cannot be transmitted, located between the second non-phase shift region and the second phase shift region, on a second mask substrate.
During the second exposure, the second phase shift region and the second non-phase shift region of the second alternating phase shift mask are made to correspond to the first non-phase shift region and the first phase shift region of the first mask, respectively. Consequently, a portion where the intensity of exposure light was weak during the first exposure because of having passed through the first alternating phase shift region (S), corresponds to the second non-phase shift region (NS) during the second exposure, so that the light intensity can be compensated. As a result, the intensities of the light passing through the phase shift regions (S) and the non-phase shift regions (NS) of the first and second alternating phase shift masks have the same values. Therefore, both the xcex94CD phenomenon and the inversion phenomenon of the critical dimension values between the phase shift region (S) and the non-phase shift region (NS) with respect to different focuses may be prevented.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.